### Introduction

These guidelines are intended for surveyors who are acquainted with GPS surveying. We outline the national coordinate systems in Great Britain, describe the national positioning infrastructure and suggest how you can specify and use ETRS89 coordinates for GPS surveys. We give you an overview of the National GPS Network active and passive stations, the National Grid Transformation OSTN02. and the National Geoid Model OSGM02. Finally, we suggest how to choose choosing the map projection that best suits your project.

### The coordinate systems used in Great Britain

Many coordinate reference systems (datums) are used by geodesists, surveyors and engineers for many different purposes. As the government agency responsible for national standards in spatial positioning in Great Britain, Ordnance Survey maintains three national coordinate systems, for which we undertake to provide national positioning infrastructure available to everybody in Great Britain. The national coordinate systems are:

**ETRS89**(European Terrestrial Reference System 1989) is our national coordinate system for 3D GPS positioning. It is a much more exacting definition of the GPS coordinate system than the better known WGS84 standard. Consequently, ETRS89 coordinates are also WGS84 coordinates, but beware that general WGS84 coordinates do not necessarily meet the ETRS89 standard. ETRS89 is the GPS coordinate system standard used for high-quality GPS surveys throughout Europe.**OSGB36 National Grid**(Ordnance Survey Great Britain 1936) is our national coordinate system for topographic mapping. It is used for Ordnance Survey mapping at all scales, and for many private topographic surveys. The OSGB36 part of the name refers to the geodetic datum (system of latitude and longitude) used, and the National Grid part refers to the map projection and grid referencing convention for eastings and northings.**ODN**(Ordnance Datum Newlyn) is our national coordinate system for heights above mean sea level (orthometric heights). It was originally based on tide gauge readings at Newlyn, Cornwall. ODN is the usual definition of height above mean sea level in mainland Britain and some islands.

A guide to coordinate systems in Great Britain

This provides technical information on the national coordinate systems used in this country.

## The national positioning infrastructure

Ordnance Survey has fully adopted GPS positioning as the basis of all three national coordinate systems listed above. OSGB36 National Grid is no longer realised by triangulation stations but by the ETRS89 positions of the National GPS Network stations in conjunction with the definitive transformation model OSTN02. Likewise, orthometric heights (defined by ODN on mainland UK) will not be realised by levelled bench marks but by GPS positioning in conjunction with the Geoid model OSGM02. All surveyors who want to take advantage of the infrastructure will therefore need access to survey-grade GPS equipment.

There are three major components of the new GPS-based national positioning infrastructure:

**The National GPS Network**, which contains over 90 active GPS reference stations of the OS Net network and about 900 passive reference stations. Using this reference network, precise ETRS89 positions are obtained from your GPS equipment.**National Grid Transformation OSTN02**– the definitive transformation between ETRS89 and OSGB36 National Grid. The National GPS Network in conjunction with OSTN02 provide the standard method of obtaining locally consistent National Grid coordinates for GPS surveyors. Occupying triangulation stations with GPS is no longer necessary.**National Geoid Model OSGM02**– the national standard precise geoid model, converting precise ETRS89 ellipsoid heights to heights above mean sea level (MSL)(ODN orthometric heights for the mainland UK). With high accuracy GPS positioning using the National GPS Network, surveyors can use OSGM02 to install their own bench marks relative to the MSL datum without levelling to Ordnance Survey bench marks.

### Specifying and using ETRS89 coordinates for GPS surveys

For GPS surveying in Europe, the European Terrestrial Reference System 1989 (ETRS89) provides a single Europe-wide coordinate reference system definition. We recommend that you should specify ETRS89 coordinates for all GPS surveys requiring better than one metre accuracy. ETRS89 is the coordinate reference system used by the Ordnance Survey National GPS Network in Great Britain, and by similar networks in many other European contries. By using the OS National GPS Network, you will be working in the ETRS89 coordinate reference system common to precise surveys throughout Europe.

You may be familiar with the GPS coordinate system WGS84. From the point of view of precise GPS surveying, we can think of WGS84 as a loose term for all GPS coordinates, which can be inconsistent with each other. If your accuracy requirement (relative to the national coordinate systems) is one metre or worse, WGS84 can be used as an adequate specification of the GPS coordinate system for your survey. However, for accuracies better than that, you need a more precise specification of the GPS coordinate system. This is the role of ETRS89. Because WGS84 is a loose term for all GPS coordinates, ETRS89 coordinates are also WGS84 coordinates, but coordinates labelled only WGS84 cannot be assumed to meet the ETRS89 standard.

By using the Ordnance Survey National GPS Network in Great Britain, you will automatically obtain ETRS89 coordinates for your GPS control stations. The benefits of specifying and using ETRS89 include:

- All your GPS coordinates will be in the European standard precise GPS coordinate system ETRS89. Your survey will be consistent with others carried out in this system throughout Europe. This might not be the case if you specify only WGS84 coordinates.
- At any time after your survey, new points consistent with your original survey can be established using the National GPS Network, even if all your survey stations have been lost.
- Your survey coordinates can be instantly transformed to OSGB36 National Grid via the national standard precise transformation OSTN02. There is no need to occupy any OSGB36 triangulation stations; nor is any locally determined transformation to OSGB36 needed.
- If you need orthometric heights of your survey stations, these can be obtained directly from ETRS89 heights using the National Geoid Model OSGM02. There is no need to observe OS bench marks. But note that long GPS observation periods may be required to determine very accurate station heights. For guidelines on obtaining the best height accuracy from GPS click here.

Networks of GPS reference stations with precise ETRS89 coordinates are available throughout much of Europe. In Great Britain, the Ordnance Survey National GPS Network is available on this web site, providing an integrated infrastruture of active and passive GPS reference stations in all parts of the country. Surveying in ETRS89 GPS coordinates requires that you position the primary control stations of your survey relative to these reference stations (which are precisely monitored in ETRS89 by OS). A primary control station might be a simple survey marker for total station surveying or levelling, or a GPS reference station (base station) for DGPS (Differential GPS) or RTK (Real Time Kinematic) GPS surveying.

The following sections describe the active and passive stations of the National GPS Network, and give some guidance on using them to position your own GPS stations.

### The national GPS network active station**s**

Since 1999, Ordnance Survey have established a national network of continuously operating reference stations (CORS) in all parts of Great Britain - the active stations of the National GPS Network. The complete network, known as OS Net, includes over 90 CORS, of which data from about 50 are made freely available on the Active RINEX data server on this website. The active stations are deployed such that any point in Great Britain, will be within 75 km of the nearest active station.

The OS Net stations all use dual-frequency geodetic quality receivers, mostly with choke ring antennae. They communicate automatically with a control centre at our head office in Southampton. This central computer collects data from each station continuously and monitors the health of the system. Daily precise ETRS89 coordinates of all active stations are computed weekly by Ordnance Survey and used to monitor the positional stability and data quality of each station.

The advantages of using the National GPS Network active stations are that you need as a minimum only one dual-frequency GPS receiver, and you don’t have to leave your survey site to obtain ETRS89 coordinates for your stations. With ETRS89 coordinates of your primary stations determined in this way, you can then use your usual rapid GPS survey methods to determine ETRS89 coordinates of a large number of points of interest relative to the primary stations. These points might be lower-order control, setting-out points, detail survey points, utility assets, vehicle trajectories, hydrographic shot points, and so on.

The Active GPS network RINEX data server on this website enables you to download RINEX format GPS data (observation and broadcast navigation files) for each active station. By entering your survey location and the start and stop times of your survey session, a set of customised RINEX files are created from the one, two or more active stations closest to your location, and delivered as a single compressed file.

National GPS Network RINEX files are post-processed with GPS data recorded at one or more of your primary survey stations with a survey-grade dual-frequency GPS receiver, using your usual GPS software.

This software must be capable of importing RINEX 2.1 format data (see FAQs). The resulting coordinates of your primary survey stations will be in the standard ETRS89 coordinate system. Ordnance Survey does not offer guidance on the use of specific GPS hardware and processing software (please refer to your software documentation or vendor).

*Typical GPS observation time as a function of distance to the active reference stations used, for three horizontal accuracy levels (2 cm, 5 cm and 10 cm standard error). This should be used as a rule of thumb only, since the choice of site, atmospheric conditions, satellite geometry, and other factors, have a major impact on accuracy achievable with a certain observation time.*

For high accuracy surveying, we recommend the use of IGS (International GNSS Service) precise satellite orbits (ephemerides) rather than the RINEX navigation files supplied with the active station data (which contain the GPS broadcast satellite orbits). These are available from the IGS web site.

Using the OS National GPS Network involves mixing antenna types. It is important to tell your software the antenna phase centre offsets to apply. Go to the Antenna offsets page.

**The national GPS network passive stations**

National GPS network passive stations are publicly accessible, geodetic quality survey marks which have been precisely coordinated in ETRS89 by Ordnance Survey. There are currently about 900 passive stations, covering Great Britain such that most places are within 20-30 km of a passive station (more in the Highlands and Islands of Scotland).

The ETRS89 coordinates and full information on all passive stations is available from the Passive GPS station database on this web site. To use National GPS Network passive stations, one or more of these stations is occupied by your survey-grade receivers while another receiver occupies your primary survey station. The data recorded is post-processed using your usual GPS software and the station coordinates supplied by OS, producing ETRS89 coordinates of your primary survey station. It is also possible to use passive stations as reference stations for real-time GPS surveys if their location is appropriate for the survey.

The advantage of OS passive GPS stations is their high density in many parts of the country, meaning that in many cases you will be surveying within 20 km of more than one passive station. This allows you to keep observation times to a minimum and obtain good results even with basic analysis software. It is also possible to use less expensive single-frequency GPS receivers to obtain ETRS89 coordinates from National GPS Network passive stations. Ordnance Survey does not offer guidance on the use of specific GPS hardware and processing software (please refer to your software documentation or vendor).

For high accuracy surveying, we recommend the use of IGS (International GNSS Service) precise satellite orbits (ephemerides) rather than the RINEX navigation files supplied with the active station data (which contain the GPS broadcast satellite orbits). These are available from the IGS web site**.**

**The National Grid transformation OSTN02**

To precisely transform between ETRS89 GPS coordinates and OSGB36 National Grid coordinates (in both directions), Ordnance Survey has developed the National Grid Transformation OSTN02. Although it will transform all GPS coordinates to National Grid (within the accuracy limitations of the original coordinates), OSTN02 is specifically designed for use with ETRS89 coordinates. To take full advantage of the accuracy offered by OSTN02, it is important to collect GPS data in ETRS89 using the National GPS Network.

This method provides National Grid coordinates which are more than accurate enough for all applications where new data must be consistent with OS mapping, including our most accurate large-scale mapping. Therefore, it is not necessary for GPS surveyors to occupy triangulation monuments to determine local transformations to National Grid coordinates. OS recommends the use of the national standard OSTN02 transformation for this purpose.

### How OSTN02 works

OSTN02 consists of two grids of shift parameters, one for eastings and one for northings. Each of these grids has a resolution of 1 km and covers an area of 1250 km by 700 km. At each intersection of these grids a shift (northing and easting) is given. Therefore the complete transformation consists of just under two million parameters. The transformation at a particular point is interpolated from the shifts given for the corners of the grid cell within which the point falls.

*Exaggerated view of the OSTN02 transformation grid (each grid cell is 1 km square). The blue arrow indicates the northing and easting shift at a particular point, interpolated from the shifts at the corners of the grid cell in which it falls. In the real transformation the shifts at adjacent grid intersections are almost identical and vary smoothly.*

The grids of shift parameters were obtained by computing the ETRS89 GPS coordinates of thousands of primary and secondary OSGB36 triangulation stations, and fitting a transformation surface to the discrepancies between projected ETRS89 coordinates and the archive coordinates of the OSGB36 stations. Therefore, OSTN02 models any local distortions in OSGB36 in detail and in any area of the country and gives you National Grid coordinates which are consistent with the OSGB36 triangulation stations in that area.

OSTN02 covers the whole of GB up to 10 km offshore. Outside this limit the shift parameters in the grid are set to zero. When transformation software encounters a zero parameter it will return an "outside transformation boundary" type error.

In software containing OSTN02, the ETRS89 GPS latitude and longitude position is first projected using the standard National Grid map projection function, to obtain an ETRS89 easting and northing position. The appropriate easting and northing shifts for this position are then interpolated from the OSTN02 transformation grids, and the shifts are applied, giving OSGB36 National Grid easting and northing. The reverse transformation procedure (OSGB36 to ETRS89) is slightly more involved because an iterative procedure is needed to discover the point at which the transformation shifts should be calculated, but the result is exact.

OSTN02 is available within various commercial software packages (ask your software vendor), through the coordinate transformer utility.

### The national geoid model OSGM02

Ordnance Survey recommend that for the highest orthometric height accuracy, users should use good quality GPS observations and processing plus the OSGM02 geoid model instead of traditional Benchmarks

If you use the National GPS Network to obtain ETRS89 GPS coordinates, the National Geoid Model OSGM02 can be used to convert these ellipsoidal heights instantly to heights above mean sea level (orthometric heights) on the Ordnance Survey height datum. Ordnance Datum Newlyn (ODN) is the standard datum for heights above sea level in mainland Great Britain and some islands. Ireland, Northern Ireland and some other islands have their own mean sea level datum. OSGM02 will convert ETRS89 ellipsoidal heights to heights above the relevant MSL datum for the specified location.

OSGM02 is the only geoid model available which converts GPS coordinates directly to orthometric heights as shown on Ordnance Survey mapping. There is no need to observe OS bench marks by GPS or levelling. However, extended GPS occupation times are required to obtain high-accuracy ETRS89 GPS heights. For instance, with a continuous four hour observation at your primary survey station using the National GPS Network active reference stations, it is possible to determine station height to 2 cm accuracy. This might be more efficient and accurate than determining station height by levelling. When accurate heights are required, it is not advisable to trust individual bench marks that might have moved since they were last levelled, in some cases many decades ago. We recommend using very accurate GPS survey with the National GPS Network active stations and the National Geoid Model OSGM02.

### How OSGM02 works

OSGM02 is very similar in design to the National Grid Transformation OSTN02. It consists of a single grid of height shift parameters representing the difference between orthometric (MSL) height and ETRS89 height, with a resolution of 1 km and covering an area of 1250 km by 700 km. At each intersection of the grids a height shift value is given. Therefore, the complete geoid model consists of over 876000 parameters. The height shift at a particular point is interpolated from the shifts given for the corners of the grid cell within which the point falls.

*Exaggerated view of how OSGM02 works (the axis at 45 degrees represents Up, each grid cell is 1 km square). The blue arrow indicates the height shift at a particular point, interpolated from the shifts at the corners of the grid cell in which it falls. In the real model the shifts at adjacent grid intersections are very similar, so the geoid model is a smooth surface.*

The grid of height shift parameters was obtained by analysing a huge archive of land and marine gravity observations, combined with a global gravity model and a digital elevation model of Britain. The model was then aligned with the local height datums as used on Ordnance Survey mapping to ensure that orthometric heights produced by OSGM02 are compatible with the bench mark network which realises the vertical coordinate systems used.

OSGM02 covers the whole of the UK and Ireland up to 10 km offshore and 2 km either side of the Northern Ireland / Republic of Ireland border. Outside this limit the shift parameters in the respective grids are set to zero. When transformation software encounters a zero parameter it will return an "outside transformation boundary" type error.

OSGM02 is freely available for use or to download onto users own PC from the coordinate transformer page.

### Choosing a map projection

### The National Grid

Having used the national GPS network to position your primary stations, ETRS89 latitude, longitude and ellipsoid height coordinates are obtained for all surveyed points from your GPS network analysis software. In order to obtain grid easting and northing coordinates, the ETRS89 fixes are projected onto a mapping grid using a map projection function.

The use of a map projection always introduces distortions into your data in the form of scale factors, which make ground distances different from plotted distances. There are also small differences between grid directions and true directions. These distortions are inevitable when the curved surface of the earth is represented on a flat plane, but for most engineering applications you can make sure the distortions are negligibly small by choosing the right map projection.

The best-known map projection in Britain is the Ordnance Survey National Grid, which is a modified Transverse Mercator map projection designed for mapping the entire country. Because the area it covers is large, the distortion of distances can be significant for engineering projects. The scale factor is zero along two north-south lines (easting = 220 000 m and easting = 580 000 m). Either side of these two lines the scale factor increases as a function of easting, causing distortions of up to half a metre over one kilometre (500 parts per million). The table below shows the scale factor for different National Grid eastings.

National Grid easting 400 km ± | 0 | 20 | 40 | 60 | 80 | 100 | 120 | 140 |

Scale distortion in cm per km | -40 | -39 | -38 | -35 | -32 | -28 | -22 | -16 |

National Grid easting 400 km ± | 160 | 180 | 200 | 220 | 240 | 260 | 280 | 300 |

Scale distortion in cm per km | -8 | 0 | +9 | +20 | +31 | +43 | +56 | +71 |

Use the National Grid projection when you need compatibility with Ordnance Survey mapping. To obtain OSGB36 National Grid coordinates from ETRS89 latitude and longitude coordinates, always use the National Grid Transformation OSTN02 (see above), which is the national standard for converting ETRS89 coordinates to National Grid. If your usual software does not contain OSTN02, obtain a separate OSTN02 utility program to perform this step, or use the Coordinate Transformer on this web site. The importance of using OSTN02 is that it not only applies to the National Grid projection and datum transformation but also models in detail the distortions present in the OSGB36 triangulation network, giving you National Grid coordinates which are consistent with those of the OSGB36 triangulation stations in the locality of your survey.

**Custom mapping grids**

#### Typical engineering site surveys

If it is necessary that distances and directions scaled off your map are as close as possible to their true values, and the National Grid scale factor in your survey location is unacceptably large, you should use a customised map projection of the conformal type. Your grid coordinates will then not fit with OS mapping. There are various types of conformal map projection, but for the extents of typical engineering surveys they are effectively identical, so keep things simple: we recommend you use the Transverse Mercator projection which is widely available in GPS and mapping software.

To create a customised Transverse Mercator projection, choose values for the following:

- The latitude and longitude of the true origin of the projection, which should be a point in the middle of your survey area. No great accuracy is needed in selecting this point.
- The eastings and northings of the false origin of the projection, which should be a point just to the south of the southerly limit of your survey and just to the west of the westerly limit. The false origin will have grid coordinates (0, 0), so putting it here ensures that all your grid coordinates are positive.
- The scale factor on the central meridian of the projection: for small areas just set this equal to 1. Making this parameter slightly less than 1 can reduce the scale distortion on the far east and west sides of the grid.
- The parameters of the reference ellipsoid: for all GPS latitudes and longitude coordinates, the reference ellipsoid is GRS80 (also known as the WGS84 ellipsoid). Its parameters are: semi-major axis a = 6378137.000 m, semi-minor axis b = 6 356 752.3141 m, and eccentricity e
^{2}= 0.006670540.

Input these parameters into any GPS/mapping software which provides a customised Transverse Mercator projection function, to convert your ETRS89 latitude and longitude coordinates to custom grid eastings and northings.

#### Large linear projects

Only for very large projects do you need to consider custom map projections other than Transverse Mercator. The Transverse Mercator is ideal for linear projects which are approximately north-south in orientation, because the project can be made to lie close the central meridian where projection distortion is zero. For linear projects which are approximately east-west in orientation, the same advantage can be gained by using a Lambert Conformal Conic projection, in which the line(s) of zero distortion run east-west. If the project has some other orientation, choose an Oblique Mercator projection, in which you can specify the direction of the central line of zero distortion, and so make it run down the middle of the linear survey. In many cases this enables you to use a single mapping grid for a large project area, while keeping scale errors within acceptable limits throughout.

For the Lambert Conformal Conic projection, the parameters for which you need to choose values are similar to those of the Transverse Mercator projection, except that the scale factor on the central meridian is replaced by the latitudes of the two east-west parallel lines of zero scale distortion. For the Oblique Mercator projection, the parameters chosen are those given for the Tranverse Mercator projection plus the azimuth of the central line of zero distortion.